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Upon your back I have you sit,

And in me is a thorn to wit

You ruin all my plans, so take a walk

Upon what's not domestic, chalk

It down to living on the wrong

Me of what trains will run upon.

'In you is a thorn, how dreadful.'
'You are often a thorn in me.'
'My claws are as sharp as thorns, but they aren't current in you.'

Riddle Answer A Side

Riddle Explanation
upon your back I have you sit - a backside, or buttocks.

and in me is a thorn to wit you ruin all my plans - to be a thorn in one's side.

so take a walk upon what's not domestic - to walk on the wild side.

chalk it down to living on the wrong me of what trains will run upon - the wrong side of the tracks.